Anatomical appropriateness of intramuscular injection at a new deltoid intramuscular injection site: comparison of nerve and arterial courses among three arm positions in healthy adult volunteers

A new deltoid intramuscular injection site has been proposed as an appropriate site for the intramuscular injection into the deltoid muscle. However, its anatomical appropriateness in the hand on ipsilateral hip position has not been established. This study aimed to compare the three arm positions in healthy adult volunteers and estimate positional changes in the nerves and accompanying arteries. Measurements were obtained in the anatomical position, the hand on the ipsilateral hip position with the thumbs positioned anteriorly, and the hand on the ipsilateral hip position with the thumbs positioned posteriorly. Using ultrasonography, the posterior circumflex humeral and deep brachial arteries were identified as surrogate axillary and radial nerves markers. The distances from the injection site to either artery did not differ significantly across the three arm positions in either males or females; however, because equivalence testing was not performed, these findings do not validate the equivalent neurovascular risk among the arm positions. Deltoid muscle thickness was greater in the anatomical position, particularly in females. The injection site moved closer to acromion and farther from the upper ends of the anterior and posterior axillary lines in the hand on the ipsilateral hip position. These findings suggest that anatomical position may provide a more favorable anatomical safety margin for intramuscular injection than the hand on the ipsilateral hip position. Position changes from the anatomical position to the hand on the ipsilateral hip position may also be accompanied by corresponding positional changes in the axillary and radial nerves and their accompanying arteries.